Modern turbine installations which are used for electricity generation in groups of power stations should be capable of being started in as short a time as possible from being stationary to the rated rotation speed in order in this way to ensure that the installation has high operational reliability. Turbine installations in particular which are used to cover a peak load are especially subject to this requirement. In order to cover peak loads in this way, compressed-air energy storage installation (“compressed-air energy storage” systems or CAES systems for short) are also being increasingly used nowadays in addition to conventional steam-turbine installations or combined gas and steam-turbine installations.
While the turbine installation is being started, and before synchronization, the generators which are used to generate electricity are normally decoupled from the external power supply system into which the electricity that is generated is emitted after synchronization. In this operating state, the generators still do not produce an adequate braking load. In general, the only braking torque acting on the turbine is the windage losses of the turbine and the bearing friction losses of the turbine, and of the generator, which are generally very low in modern installations. In consequence, the turbine is accelerated to high rotation speeds even when the fuel flow rates are low, at which windage effects occur in the rear turbine stages, because the air mass flow rate is not yet sufficiently high. The windage caused by the low air mass flow leads to abnormal mechanical and thermal loads, in particular on the turbine blades. In addition, as is described in international patent application WO 03 076 780, the low air mass flow can lead to an astatic turbine behavior.
In addition, the braking load is generally inadequate as well when these installations are being shut down, immediately after the generator has been decoupled from the external power supply system.
This problem of a lack of braking load during starting and/or shut down occurs both in the case of conventional gas turbine installations, in the case of combination power station installations, in the case of CAES installations and in the case of further turbine groups which are used for electricity generation and in which the generator that is connected to the turbine is decoupled during the starting process and/or during the shut down process from the power supply system into which the electricity that is generated is fed. This also applies to turbine elements which can be shut down and must be started or shut down independently of the main turbine, as required. In the case of steam turbines, the steam generator requirement, in particular, must also be taken into account during the starting process. In the case of air turbines, the turbine may be preceded by one or more recuperators or combustion chambers, which then lead to further restrictions during operation.
In a corresponding manner, it is often necessary to take particular care during acceleration of a turbine installation to ensure that the components which are arranged in the blade-system channel or are adjacent to the blade-system channel are raised to the operating temperature in a controlled manner, matched to one another. This is necessary in order to avoid undesirable thermal expansion and unacceptable thermal stresses in the components as a consequence of the temperature change resulting from the flow in the blade-system channel. It should therefore be possible to pass as great a flow as possible of steam or air through the entire blade-system channel of the respective group, even before synchronization.
By way of example, in this context, DE 101 16 387 A1 proposes that the steam turbine in a steam-turbine installation be preheated before the actual starting process. This allows the starting process to be carried out more quickly. To do this, steam must be taken from the steam generator in some suitable manner before the actual starting process, and must be supplied to the steam turbine. Since, however, from the financial point of view the process of steam generation should as far as possible not be started until the steam turbine installation is being started, the potential of this method for speeding up the starting process of a steam-turbine installation is limited.
In DD 1566 18, external steam is used to preheat the turbine set of a power station block with steam generator while starting the steam generator. Even if the amount of external steam required is still relatively small owing to the low braking torque of the turbine set in the unsynchronized state, this external steam must be provided from an external means, for example a further steam generator or a supply line from another process. This is generally highly costly.
As an alternative, WO 03 076 780 proposes that a static frequency converter (SFC) be connected via a shaft to the generator in order to use the frequency converter to produce a braking load, and to apply this to the shaft.
Furthermore, EP 1 289 118 A1 proposes that a rectifier exciter machine be operated as an asynchronous generator for acceleration of a turbine set, by means of which a braking torque can then be produced.
From the control engineering point of view, it is particularly complex and highly time-consuming to start and/or to shut down a turbine installation which is equipped with a main group and a connectable auxiliary group, which may also be a group element. This is the case, by way of example, in steam turbines in single-shaft combination installations with a gas turbine and a steam turbine on one shaft group and in special heating and/or extraction turbines with turbine elements which can be disconnected, or else, possibly, in single-shaft air-turbine installations. For starting and/or shut down of the turbine installation, the connectable auxiliary group is normally decoupled from the main group by means of an intermediate coupling. The auxiliary group therefore has to be started and/or shut down separately from the main group. Owing to the lack of an adequate braking torque, this is particularly difficult. In addition, during the process of starting the auxiliary group while it is decoupled from the main group, only a relatively small mass flow of the working fluid, generally air or steam, can be passed through the turbine of the auxiliary group, since it would otherwise be possible to exceed the maximum permissible rotation speed limits. This often leads to inadequate temperature matching between the working fluid and the components of the turbine adjacent to the flow channel through the turbine, so that the time to reach full load is delayed because a higher mass flow cannot be permitted until the coupled state is reached. This higher mass flow is required to warm the components of the turbine up completely, which is itself a precondition for achieving full load.
DE 500 076 proposes, just with regard to the process of shutting down a power source which is connected in series downstream from a first power source, with the power sources operating on different shafts, for the power means coming from the upstream power source still to flow through the downstream power source, and for the drive shaft to be fixed by means of a brake of any desired type. This allows the downstream power source to be shut down without having to shut down the entire installation or having to send the power medium flow via a bypass line, bypassing the downstream power source. The avoidance of the bypass line that this makes possible, or the reduction in the size of the bypass line that this makes possible, leads to a considerable reduction in the hardware complexity.